The present invention pertains to an amplifier, specifically, an operational amplifier.
FIG. 10 shows the configuration of a conventional operational amplifier. Basically, the operational amplifier comprises differential input part 10 and output part 12.
Differential input part 10 comprises a pair of p-channel MOS transistors (referred to as PMOS transistors hereinafter) 14 and 16 which are differentially connected, a constant-current source circuit 18 which is connected to the source terminals of said two PMOS transistors 14 and 16, a current-mirror circuit comprised of a pair of n-channel MOS transistors (referred to as NMOS transistors hereinafter) 20 and 22 which are connected to the drain terminals of said pair of PMOS transistors 14 and 16.
In output part 12, NMOS transistor 24, constant-current source circuit 26, and PMOS transistor 28 comprise an output stage. NMOS transistors 30, 32, and 34, constant-current source circuit 36, and PMOS transistor 38 comprise an output control part for controlling the operation of the output stage, specifically, the operation of PMOS transistor 28 used for charging the load.
Since the output terminal OUT of output part 12 is connected in series with the gate terminal of PMOS transistor 16 or the inverting input terminal (xe2x88x92), the aforementioned operational amplifier acts like a sink type voltage follower (FIG. 11).
That is, when the voltage at the input terminal IN rises from the balanced state between the voltage at the input terminal IN and the voltage at the output terminal OUT, in differential input part 10, the drain current of PMOS transistor 14 decreases, while the drain current of PMOS transistor 16 increases by the same amount, and the potential at node Na drops. As a result, the drain current of NMOS transistor 24, used as a load to sink current in the output stage of output part 12, decreases. On the other hand, in the output control part, since the potential at the gate terminal of NMOS transistor 30 (potential at node Na) drops, its drain current decreases, and the potential at the drain terminal rises. Thus, the potential at the drain terminal of NMOS transistor 32, that is, the potential at the gate terminal of NMOS transistor 34, rises, and the drain current of NMOS transistor 34 increases. The drain current of PMOS transistor 28 in the current-mirror circuit comprising PMOS transistors 38 and 28 also increases. The drain current of said PMOS transistor 28 combines with the constant-current generated by constant-current source circuit 26 at the output terminal OUT to charge the load (not shown in the figure). When the voltage at the output terminal OUT becomes equal to the voltage at the input terminal IN, the operation of each part becomes stable.
When the voltage at the input terminal IN drops from the balanced state between the voltage at the input terminal IN and the voltage at the output terminal OUT, the operation of each part is carried out in the opposite way. While the drain current of NMOS transistor 24, used as a current sink, increases, the drain current of PMOS transistor 28, used for charging, decreases, and the load is discharged by NMOS transistor 24. When the voltage at the output terminal OUT becomes equal to the voltage at the input terminal IN, the operation of each part becomes stable.
Applications requiring high-speed operational amplifiers have increased in recent years. For example, in a source driver, which drives the signal lines of a liquid-crystal display (LCD), a signal line load equivalent to tens of pF must be charged/discharged stably and reliably during the time of one horizontal scanning period (although the time varies depending on the panel system, it is about 18 xcexcs for an XGA panel with 1024xc3x97768 pixels). Consequently, the operational amplifier used for the output buffer of the driver must have extremely high driving ability (rise and fall characteristics).
Conventionally, in order to increase the speed of an operational amplifier, the transistor size of each part in the operational amplifier is increased. This method, however, is undesirable because it conflicts with another important requirement, miniaturization of the operational amplifier and the chip. In the aforementioned source driver, since many, that is, about 400, operational amplifiers are incorporated on a single chip, the chip size is greatly affected by the size of the transistors making up the operational amplifiers. Also, if the size of the transistors in each part of the operational amplifier is increased, the pn junction capacitance, etc. will be increased, which becomes the main source of oscillation. In addition, the current consumption of each part and the current consumption of the entire operational amplifier are increased significantly.
One purpose of the present invention is to solve the aforementioned problems by providing a type of amplifier which can realize high-speed rise and/or fall characteristics without increasing the transistor size.
Another purpose of the present invention is to provide a type of amplifier which can realize high-speed rise and/or fall characteristics with little increase in current consumption.
Yet another purpose of the present invention is to provide a type of amplifier which can realize high-speed operation without affecting the characteristics (such as the anti-oscillation stability) in the regular state.
In order to realize the aforementioned purposes, the present invention provides an amplifier having a differential input part for differentially inputting a pair of input signals, an output part which amplifies and outputs the output signal of the aforementioned differential input part, a constant-current source circuit which is arranged in the aforementioned differential input part or the output part, a first transistor which is turned on when the voltage difference between either of the aforementioned input signals and the output signal of the aforementioned output part exceeds a prescribed value, and a second transistor which is turned on when the aforementioned first transistor is turned on and which adds the current that results from the on-state to the constant current generated by the aforementioned constant-current source circuit.
In the amplifier of the present invention, when the voltage of a prescribed input signal input to the differential input part is higher than the voltage of the output signal of the output part by a prescribed value, the first transistor is turned on. Preferably, the input signal is supplied to one of the terminals of the first transistor, and the output signal is supplied to the control terminal of the first transistor. When the voltage difference between the two signals exceeds the threshold value of the first transistor, the first transistor is turned on. It is also possible to supply the input signal to the control terminal of the first transistor and supply the output signal to one of the terminals of the first transistor. When the voltage difference between the two signals exceeds the threshold value of the first transistor, the first transistor is turned on.
The second transistor is also turned on when the first transistor is turned on. Preferably, there is a certain direct or indirect relationship between the conducting current of the first transistor and the conducting current of the second transistor. The conducting current to the second transistor is multiplexed with or added to the constant current generated by the constant-current source circuit and supplied to the active elements of the differential input part or output part. In this way, the operating speed of each part can be increased.
In the amplifier of the present invention, the second transistor used for increasing the speed can also function appropriately to realize the same current reinforcing effect as described above with respect to the charging current of a charging circuit or the discharging current of a discharging circuit in the differential input part or the output part instead of the constant current generated by the constant-current source circuit.